The brake light switch was working because the center brake light worked. From that switch, current for the other two lights goes through the turn signal switch, and that is a common suspect. We also have to look for overheated connector terminals, but that is not a good suspect because the amount of current flow in that circuit is not sufficient to cause much trouble, and it has a small chance of affecting both the left and right circuits at the same time.
If the two rear brake lights quit at different times, a good suspect then is someone added a trailer wiring harness years ago, and spliced the wires to the truck's wires with Scotch-Lok connectors. Those do not seal out moisture, so the wires will corrode over time, and lead to dead circuits.
A corroded ground wire for the rear lights is another common suspect, but that will include the tail lights and license lights. Depending on whether one or two ground wires are used, all of the rear lights could be dead, or only all the lights on one side could be dead. The second condition can be misleading because a turn signal light can falsely appear to be working because it gets a ground, (a poor ground), through the interconnected light on the other side. The symptoms will be confusing, and quite different than what you described.
Wednesday, December 23rd, 2020 AT 1:31 PM